The present invention relates to a method for diluting acid or alkaline stock solution an apparatus therefor. More particularly, it relates to a method for diluting an acid or alkaline stock solution, which is capable of accomplishing desired dilution with higher accuracy and efficiency when, for instance, diluting a stock solution of an alkaline developer for positive resists with pure water, and an apparatus used for performing such a diluting method.
In manufacture of precision parts such as semiconductor elements, an acid or alkaline solution with a strictly controlled concentration is used. For instance, an alkaline developer such as a solution of tetramethylammonium hydroxide (TMAH) is used as a developer for positive resists, and such a developer needs to be adjusted in its concentration with tolerances of .+-.0.005 wt % against the aimed concentration (e.g. 2.38 wt %).
Conventionally, it has been common practice to transport the developer already adjusted to an aimed concentration by the developer maker to the site of use. Such a practice, however, requires a high cost for transport in the above practice, so that recently it is studied to transport the stock solution of the developer and to dilute the stock solution with pure water to an aimed concentration at the site of use. Then, researches have been done on the method and apparatus for diluting the stock solution to prepare a developer of an aimed concentration on the spot (site of use).
For instance, in Japanese Patent Publication (KOKOKU) No. 6-7910, there is described a developer stock solution-diluting apparatus in which the stock solution of an alkaline developer and pure water are supplied to a stirring tank having a mixing means, and a part of the mixed solution is extracted from the stirring tank to measure conductivity of the solution while the flow rates of the stock solution and pure water supplied to the mixing means are controlled. This diluting apparatus has the advantage which is capable of quick and real-time assay of concentration of the mixed solutions by the conductivity method (electrode method).
However, the diluting techniques which use the conductivity method, have the problems in relation to accuracy of measurement and stability, such as liability to error of the measurement due to temperature change in the electrode or enlargement of such error of measurement due to contamination of the electrode surface or deterioration of electrode performance, so that there is a limitation on precision attainable in preparation of the developer according to the said conventional diluting techniques. Thus, development of a method and apparatus capable of diluting a developer stock solution with pure water and preparing a developer of an aimed concentration at higher precision and efficiency, has been desired.
As a result of the present inventors' studies on the subject matter, it has been found that in the preparation of the alkaline developers by diluting their stock solutions, although it is possible to assay the solution concentration with higher precision by using the potentiometric titration method, the titration method takes a relatively long time for the assay, and can not immediately apply itself to the continuous control of flow rate of the stock solution and pure water supplied. Further, the present inventors' earnest studies for making the best use of the advantage of the titration method based on the knowledge derived from the above-mentioned finding that when an acid or alkaline stock solution and pure water are mixed and treated by combining the said potentiometric titration method and a specific batch mixing process, it is possible to bring the mixed solution to an aimed concentration (target concentration) in a short time and with higher precision. The present invention has been attained on the basis of the above finding.